This invention relates to a piston and cylinder for two-stroke cycle internal combustion engines in which the fuel-air mixture is precompressed in the crankcase and flows to the combustion chamber through transfer passages formed in the cylinder wall and under the control of the piston movement, and in which the piston skirt is formed with windows adjacent to the end faces of the piston pin, the latter is mounted in piston pin bosses carried by bearing brackets depending from the piston head and, when the piston is at its lower dead center, said windows register with the inlet openings of the transfer passages so that the precompressed fuel-air mixture then flows through the interior of the piston, the windows in the piston skirt, and the transfer passages into the combustion chamber, which comprises a depression volume.
To reduce the resistance to the transfer flow and to simplify the structure of two-stroke cycle internal combustion engines in which the piston controls inlet ports for the fuel-air mixture and outlet ports for the exhaust gas, it is known from German Pat. No. 571,548 and German Pat. No. 1,092,282 to provide the piston itself with means which permit of a flow of the gases along the piston. These arrangements permit of the use of a long piston skirt, affording the known advantages, even in internal combustion engines which have a relatively short stroke. In these known pistons, grooves extend from the free end of the piston skirt to points below the ring zone along the generatrices which extend through the end portions of the piston pin.
From German Pat. No. 692,211 it is also known to provide one or more windows in those portions of the piston skirt which are disposed between the bearings for the piston pin so that the mixture flows through the interior of the piston.
Whereas these known arrangements provide a transfer path which is relatively short because it does not extend throughout the height of the piston, as in the conventional pistons, but only as far as to the ring zone, the cooling action of the flowing mixture and the large quantity of heat transfer to the piston particularly in internal combustion engines operating with a high compression ratio and at high speed result in thermal stresses in the piston.
To avoid this, the last-mentioned known arrangement comprises partitions which constrain the mixture to flow along the piston head. Whereas thermal stresses are avoided and the piston is cooled owing to this feature, the resistance to flow is substantially increased thereby so that the scavenging time is prolonged. Besides, there is a limit to the cross-sectional area, particularly with small pistons designed for a high power so that relatively large bearings are required for the piston pins.
To eliminate these disadvantages it has been proposed to provide two-stroke cycle internal combustion engines in which the fuel-air mixture is precompressed in the crankcase and is transferred through a transfer passage formed in the cylinder wall and under the control of the piston movement, with a piston which has windows which, when the piston is near its lower dead center, are in register with the transfer passages so that the compressed fuel-air mixture then flows through the interior of the piston past the piston pin and through the windows in the piston skirt and the transfer passages to the scavenging ports provided at the outlet end of these transfer passages. In these pistons, the piston pin is mounted in bearing brackets depending from the piston head and the windows are disposed opposite the end faces of the piston pin (Printed German Application No. 1,476,085).
An important disadvantage of that piston resides in that the gas in the interior of the piston is not sufficiently exchanged so that hot spots and their consequences result. The cylinder head is formed with a compact depression, which forms part of the compression chamber and in dependence on the location of the spark plug is often not scavenged at all or only very incompletely scavenged by the fuel-air mixture flowing into the combustion chamber so that there is no exchange of gas or only an incomplete exchange of gas in that depression too and the residual burnt gases which precede the spark plug result in a poor starting performance, particularly when the engine is still hot. This will adversely affect not only the power output but also the composition of the exhaust gases.